Fuel cells convert a fuel and an oxidant, locally separated from each other at two electrodes, into electrical current, heat and water. The fuel can be hydrogen, methanol, or a gas rich in hydrogen. Oxygen or air serves as the oxidant. The process of energy conversion in the fuel cell is distinguished by its being largely free of pollutants and by a particularly high efficiency. For those reasons, fuel cells are gaining increasing importance for alternative power concepts, home energy supply systems and portable applications.
Membrane fuel cells such as polymer electrolyte fuel cells (PEMFC) and direct methanol fuel cells (DMFC) are suitable for many mobile and stationary areas of application because of their low operating temperatures, compact construction and power density.
Polymer electrolyte membrane (PEM) fuel cells are constructed as stacks of numerous fuel cell units. They are electrically connected in series to increase the working voltage. Each fuel cell unit contains a 5-layer membrane electrode assembly (MEA) placed between bipolar plates, also called separator plates, for gas introduction and as current leads. One such 5-layer membrane electrode assembly is, in turn, built up of a polymer electrolyte membrane that has an electrode layer on each side (3-layer catalyst-coated membrane, CCM). Then so-called gas distribution layers (GDLs) are applied to both sides of the CCM, thus producing a 5-layer membrane electrode assembly.
One of the electrode layers is made as the anode for oxidation of hydrogen, and the second electrode layer is made as the cathode for reduction of oxygen.
The gas distribution layers are usually made of carbon fiber paper or carbon fiber cloth. They allow good access of the reaction gases to the reaction layers and good conduction of the current away from the cell and the water that is produced.
The electrode layers for the anode and cathode contain a polymer that conducts protons and electrocatalysts, which catalytically support the particular reaction (oxidation of hydrogen or reduction of oxygen). Metals of the platinum group of the periodic table of the elements are preferred as catalytically active components. In most cases, so-called supported catalysts are used, in which the catalytically active platinum group metals are applied to the surface of a conductive support material such as carbon black.
The polymer electrolyte membrane comprises polymeric materials that conduct protons. These materials are also called ionomers. It is preferred to use a tetrafluoroethylene-fluorovinyl ether copolymer with sulfonic acid groups. This material can, for example, be obtained from DuPont under the tradename “Nafion®.” However, other ionomeric materials, especially some that do not contain fluorine, such as doped sulfonated polyetherketones or doped sulfonated or sulfinated arylketones or polybenzimidazoles can also be used. Suitable ionomer materials are described by the prior art. For use in fuel cells, these membranes need to have thicknesses generally between 10 and 200 μm.
Catalyst-coated membranes (3-layer CCMs) are usually prepared by applying the electrode layers to the polymer electrolyte membrane by printing, doctor blade coating, rolling or spraying, using a pasty preparation. The pasty preparations are called inks or catalyst inks and they generally contain, along with the supported catalyst, a proton-conducting material, various solvents, and optionally finely divided hydrophobic materials and pore-formers. The catalyst inks can be differentiated by the nature of the solvent used. There are inks that contain predominantly organic solvents, and inks that use predominantly water as the solvent. For instance, the prior art discloses catalyst inks that contain a mixture of water and glycolic solvents and catalyst inks in which only water is used as the solvent.
The gas distribution layers (GDLs) usually consist of coarse-pored carbon fiber paper or carbon fiber cloth with porosities of up to 90%. These materials are impregnated with hydrophobic materials, such as dispersions of polytetrafluoroethylene (PTFE) to prevent flooding of the pore system with reaction water produced at the cathode. To improve the electrical contact between the electrode layers and the gas distribution layers, those are often coated on the side toward the particular electrode layer with a “balancing layer” of carbon black and a fluoropolymer (“microlayer”). In addition, the gas distribution layers themselves can be supplied with an electrocatalyst layer. Thereby so-called catalyzed GDLs are obtained. In any case, as already discussed, one gets a 5-layer membrane electrode assembly by applying two GDLs to the two sides of a CCM.
Commercialization of the PEM fuel cell technology requires processes for mass production of catalyst-coated membranes (CCMs) and for membrane electrode assemblies so that they are available in large numbers for mobile, stationary and portable applications.
It is known in the art that one can coat the polymer electrolyte membrane using the transfer or decal process. This process uses membranes in the ion-exchanged form (e.g., the Na+ form) and yields thin catalyst coatings with layer thicknesses less than 10 μm. This process involves many steps, is tedious, expensive, and therefore suitable only for small-scale production.
Continuous processes for coating the polymer electrolyte membrane are known in the art. Some prior art references disclose a coating process for continuous production of a composite of electrode material, catalyst material and an ionomeneric membrane, in which an electrode layer on a carrier is produced from a catalytic powder comprising the electrode material, the catalyst material and the ionomeric material. This electrode layer is heated on the side away from the carrier to soften the ionomeric material and rolled onto the ionomeric membrane under pressure. The rolling process can cause damage to the ionomeric membrane and to the electrode layer.
Other prior art references disclose continuous processes for coating a polymer electrolyte membrane with electrode layers, in which a ribbon-like ionomeric membrane is pulled through a bath of a platinum salt solution. The salt that adheres is then reduced to the noble metal in a gas stream or in another bath. Selective coating, i. e., application of the electrode layer to the membrane in a desired pattern, is not possible with this process. Also, only very small amounts of catalytically active material can be applied to the membrane with this process.
Some prior art references disclose processes for continuous production of material composites in which the material composites consist of several functional materials. These composites can be used in fuel cells and fluid preparations containing the catalytic material (catalyst inks) can be used to produce the catalyst layers.
Other prior art references disclose processes for producing membrane electrode assemblies in which the polymer electrolyte membrane, the electrode layers and the gas diffusion layers are combined continuously in a rolling process.
Continuous processes are also used for selective application of electrode layers to a ribbon-shaped ionomeric membrane in which the front and back sides of the membrane are printed. For these processes, the polymer electrolyte membrane should have certain water content (less than 20%). Because of the dimensional changes of the membrane during the coating process, it is difficult to position the printing on the front and back sides accurately, especially with thin membranes (less than 50 μm thick).
Other continuous processes for coating an ionomeric membrane involve using a membrane that is pre-swollen in an organic solvent, then coated and shrinkage during drying is prevented by clamps. These processes have many problems, because the pre-swelling of the membrane cannot be controlled precisely. Because of the excess swelling and the resulting expansion of the membrane, exact positioning cannot be attained in the subsequent printing. Furthermore, the soft, rubbery ionomer membrane can easily be damaged in its swollen condition by the tension of the clamps.
The prior art also discloses processes for the manufacture of catalyst coated ion exchange membranes using a base material, where the membrane is fixed on a base material, such as polyethylene terephthalate, PTFE, or glass slides, then coated. After drying the base material is peeled off. For coating the second side the membrane is fixed on another base material by means of an adhesive tape. Thus, both first and second side coating require the use of the base material. As base materials, foils made of polymers such as PET and PTFE, but also glass slides (made of Pyrex) are disclosed.
Based on the current state of the art, there is still a need for processes that allow ionomeric membranes to be coated with a catalyst continuously on both sides with high positional accuracy and without damage to the membrane. There is also a need to process these catalyst-coated membranes (3-layer CCMs) into 5-layer membrane electrode assemblies by combining them with gas distribution layers.